A diffuser plate that scatters incoming light to various directions is widely utilized in various devices, for example, a display device such as a display, a projection device such as a projector, and various types of lighting devices. An incoming light diffusing mechanism in this diffuser plate is roughly classified into a mechanism that utilizes light refraction resulting from the surface shape of the diffuser plate and a mechanism that utilizes scattering by a substance existing inside a bulk body and having different refractive index from the circumference. One of the diffuser plates which utilize the light refraction resulting from the surface shape is what is called a microlens array diffuser plate including a plurality of microlenses having sizes of approximately several tens of micrometers and located on the surface of the bulk body.
The above microlens array diffuser plate including a plurality of microlenses located regularly has a feature of easily obtaining flat diffusion angle distribution characteristics, in the case where the diffusion characteristics of the diffuser plate are being focused on. On the other hand, such a microlens array diffuser plate has a problem that the microlenses located regularly exhibit a function like a diffraction grating, and an intense high-order diffraction component is being increased.
In order to prevent such increase of the high-order diffraction component, there are various proposals, such as irregularly locating the microlenses positioned on the surface of the bulk body, and setting variation in the diameters, the surface roughnesses, and the curvature radii of the respective microlenses.
For example, the following Patent Literature 1 discloses that a plurality of microlenses are located irregularly or with probability-distributional regularity, so as to differ from a basic pattern in which each of all the peak intervals between microlenses adjacent to each other has a regular interval L. In more detail, Patent Literature 1 discloses that each microlens is located such that the peak position of every microlens is positioned within a circle having a radius of 0.3L or less and centered at the peak position in the basic pattern, or each microlens is located such that the peak interval P between microlenses adjacent to each other satisfies 0.4L≤P≤1.6L.
Also, for example, the following Patent Literature 2 discloses that the diameters of the microlenses that compose the microlens array are each set to 100 μm or more and 1000 μm or less, and the surface roughnesses (Ra) of the microlenses are each set to 0.1 μm or more and 10 μm or less.
Also, for example, the following Patent Literature 3 discloses that a concave-convex portion is formed by and provided with a large number of concave portions and conic convex portions each having a peak portion that forms part of a sphere, a length L at the longest portion of a bottom portion of 5 μm to 100 μm, and a height of 5 μm to 100 μm, the concave portions and conic convex portions being arrayed on a transparent base material, and that a ratio r/L of a curvature radius r of the sphere of the peak portion to the length L of the longest portion of the bottom portion is set to 0.01 to 0.6.